The present invention relates to an image forming apparatus, such as a copying machine, a printer or a facsimile apparatus employing an electrophotographic method, and more particularly to an image forming apparatus which is capable of preventing an image trouble, such as disorder of a formed image defect called a "smear" occurring during a transferring operation caused from deformation of a pressuring roll of a fixing unit.
Hitherto, a portion of image forming apparatuses, such as copying machines, printers and facsimile apparatuses has a structure that a toner image formed on a photosensitive drum formed is transferred to the surface of a transfer member, such as transfer paper, by a transferring roller arranged to be brought into contact with the surface of the photosensitive drum and supplied with a bias voltage. Moreover, the toner image transferred to the transfer member is fixed with heat and pressure in a fixing unit so that an image is formed. The fixing unit provided for the foregoing image forming apparatus has a structure that a metal fixing roller including a heat source and a pressuring roller having a surface coated with an elastic layer are brought into contact with each other. Moreover, a transfer member to which a toner image has been transferred is allowed to pass through a contact portion between the fixing roller and the pressurizing roller so that the toner image is fixed to the surface of the transfer member with heat and pressure.
Since the fixing unit of the above-mentioned image forming apparatus has the structure that the fixing roller and the pressurizing roller are always in contact with each other, the portion of the elastic layer of the pressurizing roller which is in contact with the fixing roller is undesirably deformed when the operation of the image forming apparatus is interrupted. If the elastic layer of the pressurizing roller of the fixing unit is deformed, a problem arises when a transfer member to which a toner image has been transferred is fixed when a next image forming process is performed. That is, when a portion in the vicinity of the leading end of the transfer member has been introduced into the deformed portion of the pressurizing roller, change in the outer diameter of the pressurizing roller occurring because of the deformation of the pressurizing roller causes the moving speed of the transfer member to be changed (oscillated), the transfer member being moved in a state in which the transfer member is held in the contact portion between the fixing roller and the pressurizing roller. As a result, moving speed of the trailing end of the transfer member which is positioned in a transferring position of the photosensitive drum is changed. Thus, there arises a problem in that the toner image which is transferred from the photosensitive drum to the surface of the transfer member encounters disorder of an image called a "smear" occurs, as shown in FIGS. 15 and 16.
To prevent disorder of an image called a "smear" caused from the deformation of the pressurizing roller, an amount of a loop of the transfer member has been enlarged between the transferring portion of the photosensitive drum and the fixing unit. Thus, change in the moving speed of the transfer member occurring because of the deformation of the pressurizing roller exerts an influence on the transferring portion of the photosensitive drum.
However, the above-mentioned conventional technology has the following problems. That is, the foregoing conventional image forming apparatus incorporates the fixing unit having the metal fixing roller, which includes the heat source, and a pressurizing roller having the surface coated with the elastic layer. To improve the fixing performance of, in particular, small-size apparatuses, a portion of the apparatuses incorporates a thick elastic member which forms the elastic layer of the pressurizing roller. In this case, there arises a problem in that the amount of the deformation of the pressurizing roller is too large, the deformation cannot satisfactorily be restored and thus disorder of an image called a "smear" occurring because of the deformation of the pressurizing roller cannot effectively be prevented.
In recent years, size reduction of the image forming apparatus has been required by reducing the distance from the transferring portion of the Photosensitive drum to the fixing unit, that is, a short paper path has been employed. Thus, space reduction is attempted. Therefore, the amount of the loop of the transfer member cannot be enlarged between the transferring portion of the photosensitive drum and the fixing unit. What is worse, oscillations of the transfer member occurring because of the deformation of the pressurizing roller can easily be propagated to the transferring portion. Therefore, there arises a problem in that disorder of an image called a "smear" easily occurs because of the deformation of the pressurizing roller.
When a transferring roller arranged to be brought into contact with the surface of the photosensitive drum and applied with a bias voltage is employed in place of a corona type unit in order to prevent generation of ozone or the like from the transferring means, slip transference is usually employed to prevent an image defect called a "hollow character". Therefore, there arises a problem in that the force for moving the transfer member at the transferring position of the photosensitive drum is unsatisfactorily weak, an influence of oscillations of the transfer member occurring because of deformation of the pressurizing roller is easily exerted and disorder of an image called a "smear" easily occurs because of deformation of the pressurizing roller.
The image defect called a "hollow character" arises when a transferring roller 101 serving as the transferring means arranged to be brought into contact with the surface of a photosensitive drum 100 and applied with a bias voltage is used. When a toner image 102 having a somewhat large width is transferred to a transfer member 103 in the axial direction of the photosensitive drum 100 as shown in FIG. 13, the transferring roller 101 which is brought into contact with the surface of the photosensitive drum 101 is deformed, as shown in FIG. 14. Therefore, the pressing force at the central portion of the toner image is weakened, causing a phenomenon to occur in which the central portion of the toner image 102 is not transferred and white portion is undesirably generated.
To prevent occurrence of an image defect called a "hollow character", the image forming apparatus incorporating the transferring roller usually employs slip transfer with which transference is performed while the transfer member is slipped between the photosensitive drum and the photosensitive drum. Since the transfer paper is in a slippage state with respect to both of the photosensitive drum and the transferring roller, the transfer paper cannot stably be moved. Thus, there arises a problem in that image disorder called a "smear" easily occurs because of the deformation of the pressurizing roller.
Recently, image forming apparatuses have been digitized to improve the image quality and enable a variety of image processes to be performed. Thus, a portion of the apparatuses forms an image by exposing dot images to light and by developing the same. Therefore, there arises a problem in that the image disorder called a "smear" occurring because of deformation of the pressurizing roller becomes critical.
Image forming apparatuses developed recently have high processing speeds to satisfy a required for raising the speed. Since the processing speed has been raised, oscillations of the transfer member are intensified because of deformation of the pressurizing roller. Thus, there arises a problem in that image disorder called a "smear" easily occurs because of deformation of the pressurizing roller.
As a means for solving the above-mentioned problems, a method arranged as disclosed in, for example, the Unexamined Japanese Patent Application No. Sho 62-47669 may be employed, the method being arranged such that a large loop is provided for the transfer member at a position between the transferring position of the photosensitive drum and the fixing unit so as to prevent easy propagation of the oscillation of the transfer member caused from the deformation of the pressurizing roller.
However, the above-mentioned method encounters another problem in that an attempt to provide a large loop for the transfer member at the position between the transferring position of the photosensitive drum and the fixing unit causes the transfer member, to which a non-fixed toner image has been transferred, to easily be brought into contact with an adjacent member because the sizes of the image forming apparatuses have been reduced. Thus, rubbing of an image defect called a "smudge" occurs critically to actually employ the above-mentioned method.
A method arranged as disclosed in the Examined Japanese Patent Application Publication No. Sho 62-4716 may be employed, the method having a structure that the pressure applied to the fixing roller and the pressurizing roller is suspended when a copying cycle has been completed so that deformation of the pressurizing roller is prevented. In this case, a mechanism for suspending the pressure applied to the fixing roller and the pressurizing roller must be added. Thus, the cost and the size of the apparatus are enlarged. What is worse, a process for suspending the pressure applied to the fixing roller and the pressurizing roller must be performed when the copying cycle has been completed. As a result, there arises problems in that the productivity deteriorates and reliability becomes unsatisfactory because the process for pressuring the fixing roller and the pressurizing roller and the pressure suspending process must be performed.
A method arranged as disclosed in the Unexamined Japanese Patent Application publication No. Hei 7-334027 may be employed, the method being formed such that idle rotation of the pressurizing roller is performed before the image forming process is started. In the above-mentioned case, start of the image forming operation cannot quickly be performed. To restore the deformation of the pressurizing roller, the pressurizing roller must be rotated for several minutes. That is, there arises a problem in that a long waiting time is required.